Gas chromatographs are well known devices used to separate chemical mixtures. Within a gas chromatograph is an analytical column which generally comprises an elongate fused silica capillary tube coated internally with a cross bonded stationary phase. The column may be from a few tens to a few hundreds of micrometers in diameter and vary in length from a few meters to as many as a hundred meters or so. It is known to use a carrier gas in these columns such as helium, nitrogen, argon or hydrogen in order to allow transport of the separated analytes through the column and to a suitable detection system. The various gasses used as the carrier gas have differing viscosities, heat conductivities, diffusion rates and chemical activities amongst other properties.
FIG. 1 illustrates the well known Van Deemter plot showing the optimum linear velocity for various carrier gas types. As can be seen in the plot, nitrogen offers very good separation efficiency (a low value of Height Equivalent of a Theoretical Plate or HETP) at low linear velocities, but this efficiency rapidly drops as the linear velocity increases. By contrast, hydrogen can be utilized at high linear velocities without much increase in the height equivalent of a theoretical plate. For this reason, hydrogen may be used in order to achieve the best separation per unit time.
Unfortunately, while hydrogen can achieve high levels of separation power per unit time, it has the disadvantage of being more chemically reactive than nitrogen, helium or argon. It has been observed that hydrogen can be chemically reactive toward unsaturated analytes, forming compounds not present in the original sample. For example, it is known that styrene can be converted to ethyl benzene in a hot injection port in the presence of hydrogen. Additionally, the amount of conversion can vary depending on the activity of the injection port liner, rendering quantitation which is non-reproducible. It is also known that certain solvents e.g. methylene chloride can form hydrochloric acid at elevated temperatures in the presence of hydrogen. This can have a detrimental effect on the column stationary phase chemistry. The acidity as well as the polarity of the hydrochloric acid can also give non-linearity in the response factors for certain classes of compounds.
Adverse hydrogen reactivity may also be observed within an injector which is initially cool then subsequently heated rapidly such as occurs within a PTV (programmed temperature vaporization) injector. In addition to being potentially reactive toward chemical analytes of interest, the explosive nature of hydrogen is well known, and precautions need to be taken in order to prevent an unwanted explosion. These precautions may include a costly hydrogen sensor capable of disabling power to the system in the event of a large hydrogen leak.
Generally, injection ports for capillary columns control flow through the capillary column indirectly by applying pressure in accordance with the Poiseuille equation:
                                          ⅆ            V                                ⅆ            t                          =                                            π              ⁢                                                          ⁢                              r                4                                                    16              ⁢                                                          ⁢              η              ⁢                                                          ⁢              L                                ⁢                      (                                                            p                  i                  2                                -                                  p                  0                  2                                                            p                0                                      )                                              Eq        .                                  ⁢        1            in which V(t) is the volume of gas transferred as a function of time, t, pi is inlet pressure, po is outlet pressure, L is the length of the column, η is the viscosity of the gas and r is the column internal radius. The indirect control of flow using pressure in order to achieve a calculated flow is achieved by an Electronic Pressure Control (EPC) unit. It should be noted that, in this document, a distinction is made between a “transfer gas” that is used to transfer volatilized sample from a GC sample injector into a GC analytical column and a “carrier gas” which is used to: (i) facilitate separation of sample components as they differentially partition between the gas and the column stationary phase and travel through the length of the GC analytical column under the carrier gas flow and (ii) transport the separated gas components from an exit end of the GC analytical column to a detector for analysis.
FIG. 2 illustrates a conventional gas supply and exhaust system 25 that comprises an Electronic Pressure Control unit 2 interfaced to a split/splitless injector 1s. In addition to the pneumatic control elements of the EPC 2 illustrated in the accompanying drawings, the EPC 2 also comprises an electronic controller (not illustrated) such as a circuit board that is electronically coupled to the illustrated pneumatic elements and that comprises program logic that includes instructions that cause the various valves to operate and that reads the various pressure sensors. The split/splitless (SSL) injector 1s is provided for receiving injections of liquid samples from a syringe (not illustrated), for flash vaporizing the liquid samples by application of heat, for mixing the volatilized sample material with a transfer gas or carrier gas and for providing a portion of the volatilized sample material to a GC analytical column 23.
The injector temperature is typically maintained (in the case of an SSL) or rapidly heated (in the case of a PTV) to a temperature in the range of 150° C. to 450° C. during sample transfer. By contrast, the analytical column is maintained at a much lower temperature during this time—typically in the range of 40° C. to 100° C. The temperature control mechanisms of the injector and the oven are independent of one another. Transfer of volatilized sample components into the cooler GC analytical column causes these components to condense on or to be deposited in or on the stationary phase within a narrow band at the inlet end of the column. Flow of the carrier gas through the GC analytical column causes differential partitioning of the various components between the stationary phase and the gaseous mobile phase and facilitates transport of the so-separated components through the column towards its outlet end. Generally, the analytical column is housed within an oven (not shown) that is configured so as to heat air within the oven to a controlled temperature and to circulate the heated air around the column so as to provide a uniform controlled temperature along the entire length of the column.
Pneumatic control elements of the EPC such as pressure sensors, proportional valves and fixed restrictors are used to carry out the functions of gas delivery and flow control for various functions such as column pressurization, septum purge and split flow control. The carrier gas supply, e.g. helium, is introduced under pressure into a gas inlet line 7 by means of a gas fitting 3. A controlled pressure of the carrier gas is provided to the injector by the electronic pressure controller 2 which results in a controlled flow of carrier gas through column 23. A fine porosity filter 4, e.g. a stainless steel frit, removes any particulate matter that may foul operation of a proportional valve 5 that is disposed downstream in the gas inlet line 7. The proportional valve 5 maintains a setpoint pressure within the body of the injector 1s in response to measurements provided by pressure sensor 8 in order to establish a calculated flow in the analytical column 23 in accordance with the Poiseuille equation. The pressure sensor 8 provides a feedback loop to a control circuit of the electronic pressure controller. Optionally, a chemical trap 6, such as a trap comprising activated charcoal is included in the gas inlet line 7 to scrub the carrier gas of potential contaminants, e.g. hydrocarbons and/or oxygen.
As is known, generally only a portion (and, frequently, only a small portion) of the volatilized sample material and carrier gas flow actually enters the column 23. The remainder of the volatilized sample material and carrier gas flow is exhausted from the system by means of septum purge vent line 13 and, frequently, split flow vent line 14. Flow restrictor 11 and flow restrictor 18, disposed, respectively, in the septum purge vent line 13 and split flow vent line 14 maintain a pressure difference between the exhaust ports 12, 19, which are at ambient pressure, and the higher-pressure segments of the vent lines 13, 14 that are adjacent to the injector 1s. Pressure sensors 10, 17 measure the pressures of the high-pressure segments of the vent lines 13, 14. This information is provided as continuous feedback to the electronic pressure controller which calculates the pressure differences across the flow restrictors 11, 18 and operates the proportional valves 9, 16 so as to maintain desired flow rates within the vent lines 13, 14, in accordance with a mathematical calculation or calibrated lookup table. A chemical trap 15 may be included in the split flow vent line 14 to protect the proportional valve 16 from contamination by various oils and greases that may volatilize from the sample or outgas from injector components. Another similar chemical trap (not shown) may also be included in the septum purge vent line 13.
The injector 1s includes two basic modes of operation: split and splitless. In the split injection mode, a split flow is established that exits the split vent line 14. This mode of operation is used for injection of concentrated analytes to prevent overloading of the column or saturation of the detection system used at the terminal end of the column. In the splitless mode of operation, the split vent line 14 is closed (i.e., proportional valve 16 is closed) during a sample injection to cause the bulk of the sample material to be transferred to the capillary column. After a specified time interval, the split vent line is once again opened to vent residual solvent vapors and to dilute any contaminants that might outgas from contaminated surfaces.
The indirect control of flow using pressure as described above can result in a relatively unbounded flow of carrier gas if the column should break near the point where the column enters the injector. This presents a potential safety hazard associated with the conventional system 25 if hydrogen is utilized as the carrier gas. Nonetheless, the higher price and sporadic availability of helium have led to an increased market demand for hydrogen in spite the various disadvantages of the latter.
Accordingly, there is a need in the art for a gas chromatograph with a carrier gas system which allows the high separation efficiency per unit time of hydrogen without incurring the negative aspects of chemical reactivity toward analytes in a hot injector or other hot zone of a gas chromatograph. There is also a need for systems and techniques that allow hydrogen to be used as a carrier gas without formation of hydrochloric acid when injecting halogenated solvents. There is a further need to provide a hydrogen-using gas chromatograph system that provides an increased margin of safety over that provided with prior art hydrogen equipped chromatographs. The present invention addresses these needs.